Generalized condition. 
Ovipositor non-telescopic. 
Legs simple. 
Femora and tibiz with simple setze. 
Pulvilli. 
Costa around entire wing. 
Anal angle full but not lobose. 
Alula and large rimmed calypteres. 
Humeral crossvein present. 
Subhumeral crossvein. 
Full neuration. 
Veins fixed for the genus. 
Veins straight. 
Auxiliary vein distinct, ending in costa. 
First and second veins long. 
Third vein forked. 
Pedicel of second and third vein long. 
Basal cells long. 
Anal cell long, with acute end. 
Anal vein strong. 
Anal vein continuous with under side of 
anal cell. 
FAM. EMPIDID/E 5 
Specialized condition. 
Ovipositor lengthened. 
Legs armed. 
Femora and tibiz with scales, or ciliate. 
No pulvilli. 
Hind margin not thickened, anterior veins stron- 
ger, i. e. better fliers. 
Analangle weak or else lobose. 
No alula, calypteres small. 
Absence of humeral crossvein. 
No thickening under humeral crossvein. 
Neuration incomplete. 
Veins unstable within the genus, adventitious 
veins. 
Veins undulating. 
Auxiliary vein near first, evanescent at end. 
First vein shortened. 
Third vein simple or with shortening fork. 
Pedicel of second and third vein short. 
Basal cells short. 
Second basal shortened at base. 
Anal crossvein perpendicular or reflexed. 
Anal vein abbreviate. 
Anal vein distinct from under side of anal 
cell. 
While it is a general principle that an organ once functionally lost is never to be regained there 
are however several instances in the preceding list of characters that appear confusing. For instance, 
metapleural hairs occur in so many groups of flies, even when the rest of the pleura is bare, that they 
would seem to be an atavic index. Inthe Empidide the metapleurs bear setz or hairs in the genera 
centering about Empis, Clinocera and Chelipoda, all of which are specialized in their respective subfamilies. 
Moreover in the forms near Hybos and Ocydromia the pollen of the pleura becomes villous on the meta- 
pleura. These hairs probably bear some deep-seated relation to the little-understood structures at the 
base of the wing and are also influenced by the metathoracic spiracle, which, in their best developed 
form, they seem to protect. However in a single genus, e. g. Emfis, the metapleural hairs range in 
number from a bushy coarse aggregation to but two or three fine hairs. Those species of Emfis with 
but few hairs are in other respects not more primitive than Oreogeton which has a great cluster of meta- 
pleural hairs, and more likely are losing these structures as the result of a recent retrograde specialization. 
While such species will not be expected to give rise in the future to hairy forms, it is not so easy to be 
assured that some of the bare species, like Toreus or Hesferempis, possess a primitive lack of metapleural 
hairs or have lost them through secondary specialization. 
An ancient stereotyped character that has much value in classification is to be found in the 
course of the so-called anal crossvein. Originally this posterior branch of the cubitus ended indepen- 
dently in the wing margin. Then for mechanical reasons it became flexed back, narrowing, closing 
and finally shortening the anal cell. Among living diptera every transition can be found from the open 
anal cell of RAyfAus to the minute vestige of a cell, as in the Acalypterates. In many forms the anal cell 
